This invention pertains to roadway vehicle detectors, and particularly, the invention pertains to magnetometer detectors for detecting vehicles on roadways.
Present traffic vehicle detectors consist of wire loops that act as an electrical inductor, along with a capacitor, in an oscillator circuit that detects the presence or absence of a vehicle such as an automobile, truck or bus. This kind of detection system requires the wire loop to be installed below the pavement by the insertion of the loop into typically eight saw cuts into the surface of the pavement. The four-sided loop must be about four feet on a side to provide enough sensitivity to detect smaller vehicles.
The failure rate of wire loops themselves is unacceptably high. The failures are the result of pavement upheaval and the differential in coefficients of thermal expansion between the pavement material and the wire. The wire breaks when the temperatures go too high or too low. A failure of the wire loop requires the installation of a replacement loop which is offset in location with respect to the first loop which has failed. This offset location is used because it is quite difficult to repair an in-place loop. However, having to offset the replacement loop causes some loss of optimum placement which results in some loss of vehicle detection accuracy and certainty.
Traffic engineers who use wire loops for obtaining information, not only want presence information, but want to obtain other information, including vehicle count, speed, headway or direction, occupancy, and identity. Vehicle count is obtainable with a wire loop, but obtaining speed from a single loop is not feasible since speed is determined by the time it takes a vehicle to pass between two points. Two loops do not provide sufficient time resolution of passing vehicles for obtaining accurate speed indications. Headway is a spacing between vehicles in the same lane and the present loops do not have the spatial resolution to determine vehicle spacing, particular vehicles at close distances from one another, with useful accuracy. Occupancy is the measure of the presence of a vehicle in a lane, whether moving or stationery. Present wire loop detectors are poor for accurately detecting vehicles below a certain speed thereby not being always able to detect traffic that has come to a standstill. Further, wire loops also are incapable of providing information about the type of vehicle passing over the loop since the measurement coil cannot resolve the vehicle features, especially if detection signals have relatively low signal-to-noise ratio characteristics.